The present invention relates to a wireless communication system, which is a communication system employing an MCW-MIMO (Multi Code Word-Multiple Input Multiple Output) in wireless communication and relating to a system attaining cellular communication, a terminal and a base station.
As technology to enhance frequency utilization efficiency in wireless communication, the MIMO (Multiple Input Multiple Output) has been widely employed. In the MIMO, a transmitter and a receiver has a plurality of antenna elements, respectively, and by carrying out the suitable signal processing at the receiver side, a plurality of independent space channels is generated, and different data streams can be simultaneously transmitted by each of the space channels, even in the same frequency channel. Here, “data stream” means a wireless signal transmitted from each of the transmission antenna elements of the transmitter.
FIG. 1 shows an example for explaining the concept of MIMO transmission. A wireless signal transmitted from some of the antenna elements 101 of the transmitter is received by a plurality of antenna elements 102 of the receiver through different radio channels. In this case, as shown in FIG. 1, each of the antenna elements 102 of the receiver receives a plurality of wireless signals added, which signals are simultaneously transmitted from a plurality of the transmission antenna elements 101.
The receiver separates a wireless signal transmitted from each of the transmission antenna elements from the received signal, by carrying out the multi-antenna signal processing. As the multi-antenna signal processing, a weighting processing (MMSE filter) by an MMSE (Minimum Mean Square Error) algorithm has been most generally known.
The MMSE means to make the mean square error of a transmitted signal and MMSE filter output minimum, and it is known that it decreases signal power of the data streams other than a desired data stream, to almost a degree of noise power. The MMSE filter can separate the desired data stream, by carrying out the linear processing where the weighing is carried out based on MMSE criterion for the signals received by each of the receiving antenna elements, and then combining them.
As the signal processing for separating the desired data stream, SIC (Successive Interference Canceller) has also been widely known. The SIC is non-linear signal processing to enhance signal gain of the desired data stream, by canceling interference from the data streams other than the desired data stream in the received signals.
The SIC is used in combination with linear processing such as the MMSE filter. First, the received signals are passed through the MMSE filter to separate the data stream for demodulation and decoding, and then the separated data sequence is re-modulated to generate received signal components of a specific data stream contained in the received signals, by using radio channel information estimated from the received signals. The received signals after subtraction this received signal components are input again to the MMSE filter. In this way, the SINR (Signal-to-Interference and Noise Power Ratio) for the second round of output of the MMSE filter, as compared with the first round of output of the MMSE filter, because some data stream is cancelled in advance. The SINR can be enhanced by repeating this series of operations to cancel sequentially the data streams.
Contrary to the MIMO, a system equipped with a single antenna element for both the transmitter and the receiver is called SISO (Single Input Single Output). In the SISO, it is only possible to simultaneously transmit a single data stream in the same frequency channel. However, the MIMO achieves significant enhancement of wireless transmission capacity, because it is possible to transmit simultaneously a plurality of different data streams using the same frequency channel.
This technique is to be interpreted as generation of a plurality of independent space channels by carrying out the signal processing such as the MMSE and the SIC, therefore called SM (Spatial Multiplexing). This space channel is also called a spatial layer, from an image of transmission of wireless signals in parallel, in each of the layers, by dividing a radio channel to several layers.
It should be noted that it is necessary for the number of the receiving antenna elements to be equal to or more than the number of transmission data streams, in order to separate each of the transmission data streams in the MIMO. In addition, the number of data streams, which can be transmitted simultaneously, is equal to or less than the number of the transmission antenna elements. That is, the maximum number of the spatial layers, which are possible to be generated in view of the structure of the transmitter and the receiver, coincides with a smaller value between the number of the transmission antenna elements, and the number of the receiving antenna elements. However, from restriction by quality of the radio channel or the like, there may be the case where the number of spatial layer, which can be generated, becomes less than the maximum number of the spatial layers, which can be generated in view of the structure. The number of the spatial layers, which are possible to be generated practically and determined depending on radio channel states, is called Rank.
As one embodiment of the MIMO for carrying out the spatial multiplexing, the MCW-MIMO has been studied. The MCW-MIMO is a system for transmitting each of the data packets generated independently, in each of the spatial layers. On the other hand, a system for transmitting a single packet using a plurality of spatial layers is called SCW-MIMO (Single Code Word-Multiple Input Multiple Output).
FIG. 2 shows a concept of a MCW-MIMO system. In the MCW-MIMO system, transmitted data is divided first to a plurality of subpackets by a demultiplexer 201 in response to the number of the spatial layers (Rank number). The transmitted data divided to a plurality of subpackets is each subjected to coding by turbo coding or the like, or modulation processing by a coding and modulation unit 202, and then transmitted as wireless signals from separate transmission antenna elements. These wireless signals are received through independent spatial layers 203 generated by multi-antenna signal processing such as the MMSE or SIC, and are separately subjected to decoding and demodulation processing at a demodulation and decoding unit 204. Finally, a plurality of transmitted data received is put together in a multiplexer 205 to obtain the received data.
In order to explain on the MCW-MIMO still more specifically, explanation will be given here on HARQ (Hybrid Automatic Repeat Request), which is a re-transmission control system frequently to be used in the wireless communication system. It is because the MCW-MIMO is characterized in that not only coding & decoding and modulation & demodulation but also re-transmission control by the HARQ are carried out separately for a packet corresponding to each of the spatial layers.
The HARQ is a high speed re-transmission control system closed in a physical layer, and it is employed in the wireless communication system such as, for example, cdma 2000 1xEV-DO. In the HARQ, in order to increase robustness in the radio channel, a packet composed of transmitted data and, for example, error detecting code (redundancy bits) generated by turbo coding, is divided to a plurality of subpackets for wireless transmission, and is transmitted and received by a subpacket unit. Usually, transmission is carried out in the order of a transmitted data bit series and a redundancy bits sequence. For example, data is transmitted by the first subpacket, and the redundancy bits are transmitted by the subsequent subpacket.
A data transmit node waits the response from a data receive node when a subpacket is transmitted, and decides a subpacket to be transmitted next. The data receive node, in the case of success in decoding the subpacket received, responds with an ACK (Acknowledgement), and in the case of failure in the coding, responds with a NAK (Negative Acknowledgement) to the data transmit node. In this case, the data receive node waits to receive the next subpacket, storing the received subpacket for which decoding was failed.
The data transmit node transmits the second subpacket, when the NAK is received for the first subpacket from the data receive node. The data receive node combines the first and the second subpackets when the second subpacket is received, to try decoding of the received data. That is, by utilizing the error detecting code (redundancy bits) received by the second subpacket, decoding of a data part of the first subpacket already received is tried. The data transmit node returns an ACK or a NAK to the data transmit node according to the result of decoding.
The data transmit node transmits the third subpacket, in the case of receiving the NAK as the response to the second subpacket from the data receive node. In the case where the error detecting code (redundancy bits) has been divided to a plurality of subpackets, the third subpacket transmits the residual part of the error detecting code. In this case, the data receive node, which received the third subpacket, combines the first, the second and the third subpackets to try decoding of the received data.
In this way, at every subpacket receiving in the data receive node, effective coding rate becomes smaller, and possibility of decoding successfully becomes higher. This feature is called IR (Incremental Redundancy).
The data transmit node of the HARQ system is equipped with repetition function for re-transmitting a packet transmitted already, when the NAK is received for the last subpacket. Therefore, when the NAK is received for the last subpacket, the data transmit node re-transmits the subpacket groups transmitted already in the order from the first subpacket, and waits to receive the ACK from the data receive node. In the data receive node, decoding is tried after increasing signal power by combining the newly received subpacket and the subpacket group transmitted already in advance. This function is called CC (Chase Combining).
In the present description, transmission of a subpacket to be carried out by the data transmit node in receiving the NAK, is called re-transmission, irrespective of whether the subpacket is a new one, or already transmitted one.
When the ACK is received from the data receive node, the data transmit node decides successful packet transmission and transmits a new data packet to be transmitted next to the data receive node in the above subpacket form. In the case where response of the NAK from the data receive node is repeated, and the number of the subpacket re-transmissions reached the number of limit determined in advance, the data transmit node stops the re-transmission of the subpacket. In this case, it means failure of packet transmission, and decision such as packet re-transmission request or received packet discarding is carried out by an upper layer of the data receive node.
In this way, in the HARQ system, where a transmitted packet including redundancy bits is divided to several subpackets before transmission, decoding of received data at the data receive node succeeds before receiving all of the redundancy bits when a radio channel state is good, and as a result, data communication utilizing effectively a wireless resource becomes possible. In the HARQ, by arranging a dedicated channel for transmitting the ACK/NAK information in the channel group of the physical layer, control closed in the physical layer becomes possible, and this speeds up the above re-transmission control.
In the MCW-MIMO, the above HARQ re-transmission control is carried out separately in each of the spatial layers. However, the number of re-transmissions till successful packet transmission depends on a radio channel state in each of the spatial layers, therefore it is different among the spatial layers. In the case where there is still a spatial layer for which packet transmission has not succeeded yet, while packet transmission has succeeded in some of the spatial layers and the data receive node carries out the ACK response for these spatial layers, a wireless signal is not transmitted hereafter in a layer for which packet transmission has succeeded.
By 3GPP2, which is a standardization group, a wireless system using the MCW-MIMO has been proposed, as UMB (Ultra Mobile Broadband). The above re-transmission control method is defined in 2.8 MIMO Procedures of 3GPP2 C.S0084-001-0 v2.0, 4.1.3.5.7 Forward Data Channel MIMO Multi-Code Word Mode of 3GPP2 C.S0084-001-0 v2.0, 5.5.4.1.1.3.2 Forward Link Assignment Blocks of 3GPP2 C.S0084-002-0 v2.0, and 6.5 Procedures for the InUse Instance C.S0084-002-0 v2.0.
In the MCW-MIMO system as explained in the above BACKGROUND OF THE INVENTION, the HARQ re-transmission is controlled independently in each of the spatial layers. FIG. 3 shows an example of a time sequence of HARQ re-transmission control in the MCW-MIMO. In FIG. 3, time is partitioned into time frame units with a fixed duration, and after 5 frames of transmission of certain subpackets by the transmit node, the receive node carries out the ACK or NAK response for those subpackets.
In the case of NAK response, the transmit node transmits the subsequent subpackets after 3 frames of the NAK response. Therefore, the transmit node carries out transmission of the subpackets in an 8-frame cycle. In addition, in an example of FIG. 3, 4 spatial layers are present (the number of Rank is 4), and packet transmission is carried out in each of the spatial layers. In FIG. 3, SPm,n represents the n-th subpacket transmitted in the m-th spatial layer.
Explanation will be given below on procedures of packet transmission, with reference to an example of the spatial layer 0 of FIG. 3. First, in the spatial layer 0, transmission of the first subpacket 0 is carried out in the frame 0. For this subpacket 0, the receive node carries out the ACK or NAK response in a frame 5. Here, the receive node fails the decoding of the subpacket 0, and responds with the NAK. Because the transmit node receives the NAK response, it transmits the subsequent subpacket 1 in a frame 8. After that, the receive node responds with the NAK in a frame 13, and the transmit node transmits the subpacket 2 in a frame 16. When the receive node responds with the ACK in a frame 21, the transmit node recognizes successful packet transmission, and the packet transmission in the spatial layer 0 is terminated.
Because in the spatial layer 0 of FIG. 3, the ACK response is sent back from the receive node after the third subpacket transmitted by the transmit node, this is referred to that the number of re-transmissions is 3, in the present description. In an example of FIG. 3, the numbers of re-transmissions in other spatial layers are 2 for the layer 1, 5 for the layer 2, and 6 for the layer 3, respectively.
Such variation of the numbers of re-transmissions among the spatial layers is caused by difference of radio channels among the spatial layers. That is, even when packets are transmitted by using the same wireless modulation scheme, coding scheme and coding rate, and with the same transmission power between the spatial layers, the number of re-transmissions differs due to difference of radio channel quality among the spatial layers, and the number of re-transmissions for a packet transmitted in a good quality spatial layer becomes less, and on the contrary, the number of re-transmissions for a packet transmitted in a poor quality spatial layer becomes more.